Prime Minister Manmohan Singh has described the rebels as India's biggest internal security threat, and the rebels have increased attacks to protest a crackdown Singh has said the government is planning.

Four security forces were killed and two were injured Sunday when rebels blew up a jeep on a patrol in Dantewada district in Chhattisgarh state, said R.K. Vij, an inspector-general of police. The insurgents fled after an exchange of gunfire, Vij said.

The forces were with the Central Industrial Security Force, a specialized group trained to protect government-run projects. The forces were protecting the National Minerals Development Corporation.

No one claimed responsibility for the attack, but the Home Ministry in a statement blamed the killings on the Communist Party of India (Maoist), the main rebel group fighting in the region.

"The government of India strongly condemns this unprovoked and heinous attack by the Communist Party of India (Maoist) on a public sector security force that was on a routine patrol," said the statement.

Separately, government forces recovered four bodies of suspected rebels after the weekend fighting in nearby Bijapur district, Vij said.

The rebels, who say they are inspired by Chinese revolutionary leader Mao Zedong, have been fighting for more than three decades in several Indian states, demanding land and jobs for agricultural laborers and the poor. They frequently target police and government officials, whom they accuse of colluding with landlords and rich farmers.

More than 250 police and paramilitary soldiers were killed between January and August in the region, according to government statistics.